Butter a 9×5-inch loaf pan, or spray it with a nonstick baking spray.
Mash bananas in the bottom of a large bowl. (You’ll have a little over 1 cup mashed banana total.)
Whisk in melted butter, then brown sugar, egg, and vanilla.
Place baking soda, salt, cinnamon (if using), flour, and cocoa powder in a sifter or fine-mesh strainer and sift over wet ingredients. (My cocoa is almost always lumpy, so this is essential for me.)
Stir dry and wet ingredients with a spoon until just combined. Stir in chocolate chunks or chips.
Pour into prepared pan and bake 55 to 65 minutes, until a tester or toothpick inserted into the center of the cake comes out batter-free. (A melted chocolate chip smear is expected, however.)
Cool in pan for 10 to 15 minutes, then run a knife around the edge and invert it out onto a cooling rack. Serve warm or at room temperature.
The banana bread will keep for up to 4 days at room temperature. I keep mine wrapped in foil. It goes without saying that it has never actually lasted that long.
